(1) Field of the Invention
The present invention relates to a cathode ray tube (CRT) device provided with a deflection yoke, and particularly relates to a technique for reducing a magnetic field escaping as leakage from the deflection yoke.
(2) Related Art
In recent years, standards have been developed in Northern Europe in response to concerns about a low-frequency magnetic field given off by a CRT device. There is apprehension that such a magnetic field may affect the human body. Especially in Sweden, the standards, such as the MPR II and TCO standards, have been established with the aim of suppressing the magnetic field escaping from a deflection yoke or a horizontal deflection coil in particular. The magnetic field escaping as leakage from the deflection yoke or the horizontal deflection coil is referred to as the xe2x80x9cmagnetic field leakagexe2x80x9d hereinafter. To meet the leakage limits prescribed by the standards, necessary measures should be taken for the CRT device to reduce the magnetic field leakage.
There have been techniques suggested in order to reduce the magnetic field leakage. As one example of such techniques, a magnetic field is generated as a xe2x80x9ccancel magnetic fieldxe2x80x9d in the direction opposite to the magnetic field escaping as leakage from the deflection yoke. For doing so, a xe2x80x9ccancel coilxe2x80x9d is used for generating the cancel magnetic field so as to cancel the magnetic field leakage.
A CRT device using a cancel coil is disclosed in Japanese Laid-Open Patent Application No. 3-165428 (referred to as the first prior art) and No. 6-176714 (referred to as the second prior art).
For the CRT device disclosed in the first prior art, a cancel coil for reducing the magnetic field leakage is set above an upper part of a deflection yoke and a current is supplied to the cancel coil so that a cancel magnetic field is generated. FIG. 1 shows a schematic circuit diagram of a horizontal deflection coil 27 and a cancel coil 28 of the first prior art.
As shown in FIG. 1, the horizontal deflection coil 27 and the cancel coil 28 are connected in series. By the passage of a horizontal deflection current through the cancel coil 28 as well as the horizontal deflection coil 27, the cancel coil 28 can generate a cancel magnetic field that varies in accordance with the variations in the magnetic field leakage from the horizontal deflection coil 27. The cancel coil 28 is positioned so that the cancel magnetic field is generated in a proper direction to cancel the magnetic field leakage.
Meanwhile, for the CRT device disclosed in the second prior art, a cancel coil for reducing the magnetic field leakage is made up of a closed-circuit winding and set at each of upper and lower parts of a CRT so as to face a deflection yoke. FIG. 2 shows a schematic circuit diagram of a horizontal deflection coil 37 and a cancel coil 38 of the second prior art.
As shown in FIG. 2, the cancel coil 38 made up of the closed-circuit winding is set facing the horizontal deflection coil 37. With this construction, an electromotive force is produced inside the cancel coil 38 in accordance with variations in the magnetic field leakage resulting from the generation of the horizontal deflection magnetic field. By means of the electromotive force, the cancel coil 38 generates a cancel magnetic field in a proper direction so as to cancel the magnetic field leakage.
However, the CRT devices employing the techniques stated in the first and second prior arts respectively have the following problems.
As for the first prior art, the deflection current needs to pass through the cancel coil 28 that does not contribute to the horizontal deflection. Thus, power has to be unnecessarily consumed and, in addition to this, the deflection sensitivity may be deteriorated.
As for the second prior art, power does not need to be supplied to the cancel coil 38 and so the problem of the first prior art does not occur. However, the second prior art has another problem. If the magnetic field escaping as leakage from the deflection yoke is harmful to the human body, the magnetic field leakage should be reduced in front of a front panel of the CRT device, where a user is expected to be most times. However, the cancel coils 38 are set at the upper and lower parts of the CRT, facing the deflection yoke, so that the magnetic field leakage cannot be effectively reduced at a significant position where the reduction of leakage is required most. In order to reduce the magnetic field leakage at this position, the number of turns forming the cancel coil 38 may be increased. However, the increased number of turns of the cancel coil 38 may in turn adversely affect the horizontal deflection magnetic field.
Just as with the magnetic field leakage, electric field leakage is also subject to the Swedish MPR II and TCO standards. The electric field leakage is ascribable mainly to that an electric field generated due to a difference in voltage between the facing deflection coils included in the deflection yoke is given off to the outside. A technique for reducing such an electric field leakage is disclosed in, for example, Japanese Laid-Open Patent Application No. 5-207404 (referred to as the third prior art).
For the CRT device disclosed in the third prior art, a reverse voltage supplying unit is provided to supply a voltage having a reversed polarity to the waveform of the deflection voltage applied to a deflection coil. Also, an electrode is set at the top and bottom of the inner wall of the CRT at the front panel side. The reverse voltage supplying unit supplies the reverse voltage to the pair of electrodes. This enables the electrodes to generate an electric field having the reversed polarity to the VLMF (Very Low Magnetic Field) leakage (i.e., unwanted VLMF leakage). The electric field with the reversed polarity can cancel the unwanted VLMF leakage.
Using the technique of the third prior art, however, the reverse voltage supplying unit needs to be further provided. In addition to this, the magnetic field leakage cannot be reduced using this technique.
Therefore, it is a first object of the present invention to provide a CRT device that can prevent unnecessary power consumption and reduce a magnetic field leakage with a simple construction at low costs.
It is a second object of the present invention to provide a CRT device that can prevent unnecessary power consumption and reduce magnetic and electric field leakages with a simple construction at low costs.
The first object of the present invention can be achieved by a cathode ray tube device made up of: a cathode ray tube that has a front panel and a funnel; an electron gun that is set inside a neck of the funnel and projects electron beams onto an inner surface of the front panel; a deflection yoke that is set on the funnel at the neck and deflects the electron beams projected by the electron gun; and a cancel coil that has at least one closed-loop coil, makes an interlinkage with a magnetic field leakage that escapes from the deflection yoke, and generates a magnetic field in a direction so as to cancel the magnetic field leakage, wherein each closed-loop coil is set at either a first position or a second position, the first position being at a top of the cathode ray tube with a part of the closed-loop coil running along a top edge of an effective display region of the front panel, and the second position being at a bottom of the cathode ray tube with a part of the closed-loop coil running along a bottom edge of the effective display region.
With this construction, the magnetic field leakage from the CRT makes an interlinkage with the closed-loop coil, so that the magnetic field leakage can be canceled. Since the closed-loop coil is arranged along the top or bottom edge of the effective display region, the magnetic field leakage occurring at a significant position where the reduction of leakage is required most can make an interlinkage with the closed-loop coil. Consequently, the effect of canceling the magnetic field leakage can be attained at the maximum in practical terms without interfering with the image display.
It is preferable that the closed-loop coil of the cathode ray tube device further runs near right and left corners of the front panel and near an opening of the deflection yoke at a front panel side.
By doing so, the magnetic field leakage occurring in a space from the front panel to the opening of the horizontal coil at the front panel side makes an interlinkage with the closed-loop coil. As a result, the magnetic field leakage can be more effectively canceled.
The second object of the present invention can be achieved by the cathode ray tube device, wherein the closed-loop coil of the cancel coil is grounded at one point of the closed-loop coil. To be more specific, the closed-loop coil serves as a shield against the electric field leakage and so reduces the electric field escaping as leakage from the deflection yoke.
The second object of the present invention can be also achieved by a cathode ray tube device made up of: a cathode ray tube that has a front panel and a funnel; an electron gun that is set inside a neck of the funnel and projects electron beams onto an inner surface of the front panel; a deflection yoke that includes a horizontal deflection coil, and is set on the funnel at the neck and deflects the electron beams projected by the electron gun; a first coil through which a current passes, the current varying in synchronization with variations in a deflection current passing through the horizontal deflection coil; and a second coil that has at least one closed-loop coil, makes an interlinkage with any magnetic field leakage that escapes from the deflection yoke, and generates a magnetic field in a direction so as to cancel the magnetic field leakage, wherein a part of each closed-loop coil is magnetically coupled to the first coil so that an electromotive force is produced for causing a magnetic field in the same direction as the magnetic field generated through the interlinkage with the magnetic field linkage, whereby the magnetic field leakage is further canceled.
With this construction, the electromotive force is produced inside the closed-loop coil through the magnetic coupling between the closed-loop coil and the first coil through which the current varying in synchronization with the horizontal deflection current passes. By means of the electromotive force, the closed-loop coil generates the magnetic field (i.e., the cancel magnetic field) in the proper direction to further cancel the magnetic field leakage. As compared with a case where the closed-loop coil is not magnetically coupled to the first coil, a stronger cancel magnetic field can be generated. In addition, the strength of the cancel magnetic field can be easily adjusted by adjusting the strength of the magnetic coupling.
It is preferable that the part of the closed-loop coil of the cathode ray tube device is set around the correction coil for a magnetic coupling to the correction coil. By doing so, the magnetic coupling between the closed-coil loop and the differential coil can be easily achieved. The strength of the cancel magnetic field can be adjusted by changing the number of turns of the closed-loop coil to be set around the first coil.